The present invention relates to a position adjustment device and, in particular, to a position adjustment device using a piezoelectric displacement device which deforms in response to an applied voltage to precisely adjust an object to a reference position.
Position alignment or adjustment devices have been used in various kinds of technical fields in order to precisely adjust the position of an object, such as a semiconductor wafer in an electron beam fine pattern exposure system, an optical coupling element in an optical communication system, etc. One such device is known which is used to adjust the position of a semiconductor wafer set in the pattern exposure apparatus in such a manner that the surface of the wafer coincides with the known reference surface of a sample table unit. This position alignment of the semiconductor wafer should be carried out with high accuracy. Otherwise, an image of the mask pattern will not be just-focussed onto the wafer, and it will be impossible to transfer the electron beam pattern onto the wafer precisely.
In a conventional pattern transfer apparatus using electron beam, a semiconductor wafer is position-adjusted manually by an operator and then fixed to the last determined position by using mechanical fixation means. More specifically, the semiconductor wafer, onto which the mask pattern is to be transferred, is placed within a wafer supporting member, known as "wafer cassette holder", and then put on a known table plate defining a reference surface. The wafer holder is arranged such that it can move up and down on the table to vary the height of the wafer surface with respect to the reference surface. The height of the wafer holder is adjusted to cause the wafer surface, onto which a beam pattern will be transferred, to be in precisely coincidence with the reference surface. The wafer holder is then locked mechanically, so that the wafer is held at the same level as the reference surface.
A plurality of rods are movably provided in the cylindrical holes cut in the wafer cassette holder. They can vertically move in the holes and serve as support legs for the wafer holder set on the table plate. When the rods project downward from the holes to push the table surface, the wafer holder may be lifted up on the table. The wafer holder may fall down when these rods are moved back and pulled into the holes. Therefore, by changing the distance the movable rods move from the holes, which corresponds to the leg length of the wafer, the wafer surface can be precisely aligned with the reference surface of the sample table.
After the position adjustment is completed, each rod should be rigidly clamped in the corresponding hole to hold the wafer cassette holder at the finally adjusted height. Screws are usually provided in the wafer holder for clamping the rods inside the holes. For such the clamping mechanism, it is very difficult to keep holding the wafer accurately at the last adjusted height while the screws are being rotated to compress and lock the leg rods. This is because the mechanical vibration caused by rotating the screws makes the surface of the wafer set in the cassette holder move from the justadjusted position as it is applied to the wafer cassette holder. Although the rods are being compressed by screws, the precise position adjustment cannot be achieved if the wafer holder is inclined or misaligned again with respect to the reference surface due to the mechanical vibration. An adjustment of the wafer position on the order of microns or, more precisely, of submicrons, is far from possible.